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Abstract:

An LTE base station (100) conducts radio communication with a radio
terminal which can execute a handover between a plurality of radio
communication systems having different attainable communication rates.
The LTE base station (100) is provided with a service detection unit
(121A), which detects the classification of a communication service being
used by a radio terminal, and a transmission unit (111), which sends the
radio terminal a command for handover to the radio base station of a
radio communication system, which handles the communication rate required
for the communication service detected by the service detection unit
(121A).

Claims:

1. A radio base station that performs radio communication with a radio
terminal capable of performing handover among a plurality of radio
communication systems in which attainable communication rates are
different from each other, comprising: a detection unit configured to
detect classification of a communication service being used by the radio
terminal; and a transmission unit configured to transmit a command for a
handover toward a radio base station of a radio communication system
supporting a communication rate required in the classification of the
communication service detected by the detection unit, to the radio
terminal.

2. The radio base station according to claim 1, further comprising: a
reception unit configured to receive information indicating radio quality
between the radio terminal and another radio base station from the radio
terminal, and wherein the transmission unit transmits a command for a
handover toward said another radio base station to the radio terminal,
when the radio quality is good and said another radio base station is the
radio base station of the radio communication system supporting the
communication rate required in the classification of the communication
service detected by the detection unit.

3. A radio base station that performs radio communication with respect to
a radio terminal capable of performing handover among a plurality of
radio communication systems in which attainable communication rates are
different from each other, comprising: a measurement unit configured to
measure a communication rate of radio communication between the radio
terminal and the radio base station; and a transmission unit configured
to transmit a command for a handover toward a radio base station of a
radio communication system supporting the communication rate measured by
the measurement unit, to the radio terminal.

4. The radio base station according to claim 3, further comprising: a
reception unit configured to receive information indicating radio quality
between the radio terminal and another radio base station from the radio
terminal, and wherein the transmission unit transmits a command for a
handover toward said another radio base station to the radio terminal,
when the radio quality is good and said another radio base station is the
radio base station of the radio communication system supporting the
communication rate measured by the measurement unit.

5. A handover control method, comprising: a step of detecting by a radio
base station, which performs radio communication with respect to a radio
terminal capable of performing handover among a plurality of radio
communication systems in which attainable communication rates are
different from each other, classification of a communication service
being used by the radio terminal; and a step of transmitting a command
for a handover toward a radio base station of a radio communication
system supporting a communication rate required in the classification of
the communication service, which is detected in the detection step, from
the radio base station to the radio terminal.

6. A handover control method, comprising: a step of measuring by a radio
base station, which performs radio communication with respect to a radio
terminal capable of performing handover among a plurality of radio
communication systems in which attainable communication rates are
different from each other, a communication rate of radio communication
between the radio terminal and the radio base station; and a step of
transmitting a command for a handover toward a radio base station of a
radio communication system supporting the communication rate measured in
the measurement step, from the radio base station to the radio terminal.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a radio base station that controls
handover performed by a radio terminal, and a handover control method.

BACKGROUND ART

[0002] In recent years, with the spread of radio communication and the
diversification of a communication service, since traffic transmitted in
a radio communication system is rapidly increased, it is necessary to
increase a communication rate in the radio communication system. In order
to cope with such a request, 3GPP (3rd Generation Partnership Project),
which is a standardization project of the radio communication system, has
standardized an LTE (Long Term Evolution) system regarded as the next
generation (the 3.9th generation). In such a next-generation radio
communication system, it is possible to achieve a higher communication
rate as compared with an existing radio communication system.

[0003] In the LTE system, in order to efficiently use a limited radio
resource, a function called RRM (Radio Resource Management) is adopted
(see Non-Patent Document 1). In the RRM, when the number of radio
terminals connected to a radio base station is large and load of the
radio base station is increased, the radio base station transmits a
handover command for a handover toward another radio base station to the
radio terminal. In this way, load distribution may be achieved between
the radio base stations.

[0004] Furthermore, the radio base station of the LTE system may transmit
a command for a handover toward an existing radio communication system
(for example, GSM, W-CDMA, CDMA 2000 and the like) from the LTE system to
the radio terminal.

[0006] Meanwhile, since a radio base station compatible with the
next-generation radio communication system has been gradually installed,
a communication coverage of the next-generation radio communication
system is limited when a service initially starts.

[0007] However, when a radio terminal using a communication service (for
example, a voice communication service) requiring a low communication
rate is connected to the radio base station of the next-generation radio
communication system, a high communication rate of the next-generation
radio communication system may not be provided and a radio resource of
the next-generation radio communication system may be wasted. Therefore,
there is a problem that it is not possible to effectively utilize a radio
communication system capable of a high-speed communication, as with the
next-generation radio communication system.

[0008] Therefore, an object of the present invention is to provide a radio
base station which can effectively utilize a radio communication system
capable of performing high-speed communication, and a handover control
method, when a plurality of radio communication systems exist.

[0009] To solve the above problem, the present invention has following
features. A first feature of the present invention is summarized as a
radio base station (LTE base station 100) that performs radio
communication with a radio terminal (radio terminal 200) capable of
performing handover among a plurality of radio communication systems in
which attainable communication rates are different from each other,
comprising: a detection unit (service detection unit 121A) configured to
detect classification of a communication service being used by the radio
terminal; and a transmission unit (transmission unit 111) configured to
transmit a command for a handover toward a radio base station of a radio
communication system supporting a communication rate required in the
classification of the communication service detected by the detection
unit, to the radio terminal.

[0010] According to such a feature, when the communication rate required
for each type of communication service used by the radio terminal is low,
it is possible to make the radio terminal to perform a handover toward a
radio communication system with a lower communication rate. Moreover,
when the communication rate required for each type of the communication
service used by the radio terminal is high, it is possible to make the
radio terminal to perform a handover toward a radio communication system
capable of a high-speed communication.

[0011] Therefore, it is possible to provide that the radio resource of a
radio communication system (i.e., a next-generation radio communication
system) capable of a high-speed communication is preferentially used for
the radio terminal that is using a communication service requiring a
high-speed communication rate. This enables effective use of a radio
communication system capable of a high-speed communication.

[0012] In the first feature, the radio base station further comprising a
reception unit (reception unit 112) configured to receive information
indicating radio quality between the radio terminal and another radio
base station from the radio terminal, and wherein the transmission unit
transmits a command for a handover toward said another radio base station
to the radio terminal, when the radio quality is good and said another
radio base station is the radio base station of the radio communication
system supporting the communication rate required in the classification
of the communication service detected by the detection unit.

[0013] A second feature of the present invention is summarized as a radio
base station that performs radio communication with respect to a radio
terminal (radio terminal 200) capable of performing handover among a
plurality of radio communication systems in which attainable
communication rates are different from each other, comprising: a
measurement unit (communication rate measurement unit 121B) configured to
measure a communication rate of radio communication between the radio
terminal and the radio base station; and a transmission unit
(transmission unit 111) configured to transmit a command for a handover
toward a radio base station of a radio communication system supporting
the communication rate measured by the measurement unit, to the radio
terminal.

[0014] According to such a feature, when the communication rate of the
radio communication with the radio terminal is high, it is possible to
make the radio terminal to perform a handover toward a radio
communication system capable of a high-speed communication. Further, when
the communication rate of the radio communication with the radio terminal
is low, it is possible to make the radio terminal to perform a handover
toward a radio communication system with a low communication rate.

[0015] Therefore, it is possible to provide that the radio resource of a
radio communication system (e.g., a next-generation radio communication
system) capable of a high-speed communication is preferentially used for
the radio terminal that needs to perform a high-speed radio communication
(i.e., a radio terminal that is using a communication service requiring a
high-speed communication rate). This enables effective use of a radio
communication system capable of a high-speed communication.

[0016] In the second feature, the radio base station further comprising: a
reception unit (reception unit 112) configured to receive information
indicating radio quality between the radio terminal and another radio
base station from the radio terminal, and wherein the transmission unit
transmits a command for a handover toward said another radio base station
to the radio terminal, when the radio quality is good and said another
radio base station is the radio base station of the radio communication
system supporting the communication rate measured by the measurement
unit.

[0017] A third feature of the present invention is summarized as a
handover control method, comprising: a step of detecting by a radio base
station, which performs radio communication with respect to a radio
terminal capable of performing handover among a plurality of radio
communication systems in which attainable communication rates are
different from each other, classification of a communication service
being used by the radio terminal; and a step of transmitting a command
for a handover toward a radio base station of a radio communication
system supporting a communication rate required in the classification of
the communication service, which is detected in the detection step, from
the radio base station to the radio terminal.

[0018] A fourth feature of the present invention is summarized as a
handover control method, comprising: a step of measuring by a radio base
station, which performs radio communication with respect to a radio
terminal capable of performing handover among a plurality of radio
communication systems in which attainable communication rates are
different from each other, a communication rate of radio communication
between the radio terminal and the radio base station; and a step of
transmitting a command for a handover toward a radio base station of a
radio communication system supporting the communication rate measured in
the measurement step, from the radio base station to the radio terminal.

[0019] According to the characteristics of the present invention, it is
possible to provide a radio base station which can effectively utilize a
radio communication system capable of performing high-speed
communication, and a handover control method, when a plurality of radio
communication systems exist.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a diagram illustrating the schematic configuration of a
communication system according to a first embodiment and a second
embodiment of the present invention.

[0021]FIG. 2 is a block diagram illustrating the configuration of an LTE
base station according to the first embodiment of the present invention.

[0022]FIG. 3 is a diagram illustrating a configuration example of a table
held by a storage unit of the LTE base station according to the first
embodiment of the present invention.

[0023]FIG. 4 is a flowchart illustrating the operation of the LTE base
station according to the first embodiment of the present invention.

[0024]FIG. 5 is a block diagram illustrating the configuration of an LTE
base station according to the second embodiment of the present invention.

[0025]FIG. 6 is a diagram illustrating a configuration example of a table
held by a storage unit of the LTE base station according to the second
embodiment of the present invention.

[0026]FIG. 7 is a flowchart illustrating the operation of the LTE base
station according to the second embodiment of the present invention.

[0027]FIG. 8 is a block diagram illustrating the configuration of an LTE
base station according to a third embodiment of the present invention.

[0028]FIG. 9 is a diagram illustrating a configuration example of a table
held by a storage unit of the LTE base station according to the third
embodiment of the present invention.

[0029]FIG. 10 is a sequence diagram illustrating an operation sequence of
the LTE base station and a radio terminal according to the third
embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

[0030] Next, with reference to the accompanying drawings, a first
embodiment, a second embodiment, a third embodiment, and other
embodiments of the present invention will be described. It is to be noted
that the same or similar reference numerals are applied to the same or
similar parts through the drawings in the following embodiments.

(1) First Embodiment

[0031] In the first embodiment, (1.1) Configuration Example of
Communication System, (1.2) Configuration of LTE Base Station, (1.3)
Operation of LTE Base Station, and (1.4) Operation and Effect will be
described.

(1.1) Configuration Example of Communication System

[0032] The communication system illustrated in FIG. 1 includes an LTE base
station 100, a radio terminal 200, an LTE base station 300, a CDMA base
station 400, a GSM base station 500, a communication network 600, and a
communication destination device 700. The radio terminal 200 has
established a radio connection to the LTE base station 100, and
communicates with the communication destination device 700 such as a
server or a communication terminal via the LTE base station 100 and the
communication network 600. In the example of FIG. 1, only one radio
terminal 200 is connected to the LTE base station 100. However, actually,
a plurality of radio terminals 200 are connected to the LTE base station
100.

[0033] The LTE base station 100 and the LTE base station 300 are
configured according to LTE (E-UTRAN) regarded as the 3.9th-generation
portable telephone system. In the LTE system, OFDM (Orthogonal Frequency
Division Multiplexing) is employed for a downlink and SC-FDMA
(Single-Carrier Frequency-Division Multiple Access) is employed for an
uplink. In the LTE system, a high communication rate of about 100 Mbps (a
theoretical download speed) at maximum is provided.

[0034] In addition, in the LTE system, handover led by a base station is
employed. That is, the LTE base station 100 specifies a radio base
station (a handover destination) and allows the radio terminal 200 to be
handed over to the radio base station.

[0035] The CDMA base station 400 is configured according to a CDMA system
such as W-CDMA (Wideband Code Division Multiple Access) or CDMA-2000,
which is regarded as the 3rd-generation portable telephone system. A
communication rate attainable by the CDMA system is lower than a
communication rate attainable by the LTE system.

[0036] The GSM base station 500 is configured according to GSM (Global
System for Mobile Communications) which is regarded as the 2nd-generation
portable telephone system. A communication rate attainable by the GSM
system is lower than the communication rate attainable by the CDMA
system. In detail, in the GSM system, a communication rate of about 171
kbps (a theoretical downstream value) at maximum is provided.

[0037] The radio terminal 200 is a multi-mode terminal supporting the
radio communication system such as the LTE system, the CDMA system, and
the GSM system.

[0038] The radio terminal 200 uses a communication service such as a voice
communication service, a WEB browsing service, or a file transfer
service. The "communication service" is an application software
corresponding to the uppermost layer in a communication protocol stack.

[0039] The voice communication service is a communication service not
requiring a high communication rate. The WEB browsing service is a
communication service requiring a communication rate higher than that of
the voice communication service. The file transfer service is a
communication service requiring a communication rate higher than that of
the WEB browsing service.

[0040] The radio terminal 200 being connected to the LTE base station 100
measures the reception quality (hereinafter, referred to as radio
quality) of radio signals, which are transmitted by the LTE base station
100, the LTE base station 300, the CDMA base station 400, and the GSM
base station 500, in a periodic manner or according to an instruction of
the LTE base station 100. Here, the radio quality indicates a received
signal strength indicator (RSSI), a signal to interference noise ratio
(SINR) and the like.

[0041] The radio terminal 200 transmits a measurement result report
including information on the measured radio quality to the LTE base
station 100. The measurement result report is referred to as a
"Measurement Report" in the LTE system.

[0042] The LTE base station 100 transmits a handover command, which gives
an instruction to switch a connection destination from the LTE base
station 100 to another radio base station, to the radio terminal 200
based on the measurement result report received from the radio terminal
200. For example, when the radio quality between the radio terminal 200
and the another radio base station is higher than the radio quality
between the radio terminal 200 and the LTE base station 100, the LTE base
station 100 may transmit a handover command to the another radio base
station to the radio terminal 200.

[0043] In the first embodiment, the LTE base station 100 decides a radio
base station (a handover destination) according to a communication
service being used by the radio terminal 200, as well as the radio
quality.

(1.2) Configuration of LTE Base Station

[0044]FIG. 2 is a block diagram illustrating the configuration of the LTE
base station 100 according to the first embodiment.

[0045] As illustrated in FIG. 2, the LTE base station 100 includes an
antenna unit ANT, a transmission/reception unit 110, a control unit 120,
a storage unit 130, and a wired communication unit 140.

[0046] The transmission/reception unit 110 is configured by an RF circuit,
a BB circuit and the like, and transmits/receives a radio signal via the
antenna unit ANT. The transmission/reception unit 110 includes a
transmission unit 111 configured to transmit the radio signal, and a
reception unit 112 configured to receive the radio signal.

[0047] The transmission unit 111 encodes and modulates a transmission
signal from the control unit 120, up-converts and amplifies the modulated
transmission signal, and transmits the up-converted and amplified signal
from the antenna unit ANT.

[0048] The reception unit 112 amplifies and down-converts a reception
signal from the antenna unit ANT, demodulate and decode the
down-converted reception signal, and input the demodulated and decoded
signal to the control unit 120. The reception unit 112 receives the
measurement result report transmitted by the radio terminal 200.

[0049] The control unit 120 is configured by, for example, a CPU, and
controls various functions of the LTE base station 100 and performs
various types of computation processing of the LTE base station 100. The
storage unit 130 is configured by, for example, a memory, and stores
various pieces of information used for the control, computation process
and the like of the LTE base station 100. The wired communication unit
140 communicates with another apparatus via the communication network
600.

[0050] The control unit 120 includes a service detection unit 121A and a
handover command generation unit 122.

[0051] The service detection unit 121A detects the classification of a
communication service being used by the radio terminal 200. A method for
detecting the classification of the communication service includes a
method for capturing packets exchanged by the radio terminal 200 and
analyzing each packet (in detail, a header part of a packet). However,
other methods may also be used. As other methods, there exist a
determination method based on the size of a packet and a determination
method based on session classification.

[0052] When the measurement result report received in the reception unit
112 represents that there exists another radio base station with good
radio quality with the radio terminal 200 and the another radio base
station is a radio base station of a radio communication system
supporting a communication rate required in the classification of the
communication service detected by the service detection unit 121A, the
handover command generation unit 122 generates a handover command for a
handover toward the another radio base station. The transmission unit 111
transmits the generated handover command to the radio terminal 200.

[0053] The storage unit 130 holds a table indicating a correspondence
relation between the classification of the communication service and the
radio communication system, and the handover command generation unit 122
decides a radio communication system (a handover destination) from the
classification of the communication service detected by the service
detection unit 121A, with reference to the table.

[0054]FIG. 3 is a diagram illustrating a configuration example of the
table held by the storage unit 130.

[0055] The voice communication service is associated with the GSM system.
When the voice communication service, specifically, ROHC (Robust Header
Compression) is used, the size of one packet is about 30 bytes (when a
voice codec is G.729). Thus, since a required communication rate is a low
speed (for example, about 40 kbps), it is possible to sufficiently
maintain service quality even in the GSM system.

[0056] The WEB browsing service using a protocol such as HTTP (Hyper Text
Transfer Protocol) is associated with the CDMA system. In the WEB
browsing service, since it is possible to use a comfortable service if a
medium communication rate of about 1 Mbps is generally maintained, the
CDMA system with a communication rate lower than that of the LTE system
is connected to the WEB browsing service.

[0057] The file transfer service using a protocol such as FTP (File
Transfer Protocol) is associated with the LTE system. The file transfer
service is so provided to be preferentially connected to a wideband
system such as the LTE system because the service quality is improved as
a communication rate is increased. Furthermore, it may also be assumed
that one user (terminal) simultaneously uses a plurality of services. In
such a case, since large capacity radio communication is required, the
wideband system such as the LTE system is preferentially connected to the
large capacity radio communication.

(1.3) Operation of LTE Base Station

[0058]FIG. 4 is a flowchart illustrating the operation of the LTE base
station 100 according to the first embodiment. The operation flow
illustrated in FIG. 4 is performed for each of the plurality of radio
terminals 200 connected to the LTE base station 100.

[0059] In step S100, the reception unit 112 receives a measurement result
report from the radio terminals 200.

[0060] When the measurement result report received in the reception unit
112 includes information on a radio base station with good radio quality
between the radio terminals 200 and the radio base station (step S101;
YES), the service detection unit 121A detects the classification of a
communication service being used by the radio terminals 200 in step S102.

[0061] Firstly, the case in which the radio base station with the good
radio quality is the GSM base station 500 will be described. When the
radio base station with the good radio quality is the GSM base station
500 and the classification of the communication service detected by the
service detection unit 121A is a voice communication service (step S103;
YES), the handover command generation unit 122 generates a handover
command for a handover toward the GSM base station 500 in step S106. The
transmission unit 111 transmits the handover command generated by the
handover command generation unit 122 to the radio terminal 200.

[0062] Meanwhile, when the radio base station with the good radio quality
is the GSM base station 500 and the classification of the communication
service detected by the service detection unit 121A is not the voice
communication service (step S103; NO), the handover command generation
unit 122 omits the generation of the handover command and the
transmission unit 111 omits the transmission of the handover command.

[0063] Secondly, the case in which the radio base station with the good
radio quality is the CDMA base station 400 will be described. When the
radio base station with the good radio quality is the CDMA base station
400 and the classification of the communication service detected by the
service detection unit 121A is a WEB browsing service (step S104; YES),
the handover command generation unit 122 generates a handover command for
a handover toward the CDMA base station 400 in step S106. The
transmission unit 111 transmits the handover command generated by the
handover command generation unit 122 to the radio terminal 200.

[0064] Meanwhile, when the radio base station with the good radio quality
is the CDMA base station 400 and the classification of the communication
service detected by the service detection unit 121A is not the WEB
browsing service (step S104; NO), the handover command generation unit
122 omits the generation of the handover command and the transmission
unit 111 omits the transmission of the handover command.

[0065] Thirdly, the case in which the radio base station with the good
radio quality is the LTE base station 300 will be described. When the
radio base station with the good radio quality is the LTE base station
300 and the classification of the communication service detected by the
service detection unit 121A is a file transfer service (step S105; YES),
the handover command generation unit 122 generates a handover command for
a handover toward the LTE base station 300 in step S106. The transmission
unit 111 transmits the handover command generated by the handover command
generation unit 122 to the radio terminal 200.

[0066] Meanwhile, when the radio base station with the good radio quality
is the LTE base station 300 and the classification of the communication
service detected by the service detection unit 121A is not the file
transfer service (step S105; NO), the handover command generation unit
122 omits the generation of the handover command and the transmission
unit 111 omits the transmission of the handover command.

(1.4) Operation and Effect

[0067] When a communication rate required in the classification of the
communication service being used by the radio terminal 200 is low, the
LTE base station 100 allows the radio terminal 200 to be handed over to
the CDMA system or the GSM system. When the communication rate required
in the classification of the communication service being used by the
radio terminal 200 is high, the LTE base station 100 may allow the radio
terminal 200 to be handed over to the LTE system.

[0068] In this way, it is possible to reduce the load of the LTE base
station 100 when the radio terminal 200 having used a low speed
communication service is connected to the LTE base station 100. The load
of the LTE base station 100 is reduced, resulting in the improvement of
service quality when the radio terminal 200 having used a high-speed
communication service is connected to the LTE base station 100.

[0069] As described above, it is possible to allow a radio resource of the
LTE system to be preferentially used by the radio terminal 200 having
used the high-speed communication service, so that the radio resource of
the LTE system can be effectively utilized. Moreover, handover from the
LTE system to an existing radio communication system (the CDMA system or
the GSM system) is actively performed, so that load distribution can be
more flexibly achieved.

[0070] In the first embodiment, when radio quality between a radio base
station (another radio base station) other than the LTE base station 100
and the radio terminal 200 is good, the LTE base station 100 sets the
another radio base station as a handover destination candidate of the
radio terminal 200. Since the number of radio base stations of an
existing radio communication system is large, it is easy to search for a
handover destination candidates with good radio quality.

[0071] Furthermore, when the another radio base station is a radio base
station of a radio communication system supporting the classification of
a communication service detected by the service detection unit 121A, the
LTE base station 100 transmits a command for a handover toward the
another radio base station to the radio terminal 200. In this way, it is
possible to allow the radio terminal 200 to be handed over to a radio
base station with good radio quality between the radio base station and
the radio terminal 200, and it is possible for the radio terminal 200 to
stably perform communication even after the radio terminal 200 is handed
over to the radio base station.

(2) Second Embodiment

[0072] In the first embodiment, a radio base station (a handover
destination) is decided according to both the classification of a
communication service being used by the radio terminal 200 and radio
quality. In a second embodiment, instead of the classification of the
communication service, a communication rate of radio communication with
the radio terminal 200 is used.

[0073] In the second embodiment, (2.1) Configuration of LTE Base Station,
(2.2) Operation of LTE Base Station, and (2.3) Operation and Effect will
be described, and the description overlapping the first embodiment will
be omitted.

(2.1) Configuration of LTE Base Station

[0074]FIG. 5 is a block diagram illustrating the configuration of the LTE
base station 100 according to the second embodiment.

[0075] As illustrated in FIG. 5, the LTE base station 100 includes a
communication rate measurement unit 121B, instead of the service
detection unit 121A described in the first embodiment. The communication
rate measurement unit 121B measures a communication rate of radio
communication with the radio terminal 200. The communication rate of the
radio communication with the radio terminal 200 is defined according to
the classification of a communication service used by the radio terminal
200. In order to exclude the influence by a temporary reduction of a
communication rate, it is preferable that the communication rate
measurement unit 121B measures an average value (a moving average value)
of a communication rate for a predetermined period, other than an
instantaneous value of a communication rate.

[0076] The handover command generation unit 122 generates a command for a
handover toward a radio base station of a radio communication system
supporting the communication rate measured by the communication rate
measurement unit 121B. The transmission unit 111 transmits the generated
handover command to the radio terminal 200.

[0077]FIG. 6 is a diagram illustrating a configuration example of the
table held by the storage unit 130. The GSM system is associated with a
communication rate "low speed". The CDMA system is associated with a
communication rate "medium speed". The LTE system is associated with a
communication rate "high speed".

(2.2) Operation of LTE Base Station

[0078]FIG. 7 is a flowchart illustrating the operation of the LTE base
station 100 according to the second embodiment. The operation flow
illustrated in FIG. 7 is performed for each of the plurality of radio
terminals 200 connected to the LTE base station 100.

[0079] In step S200, the communication rate measurement unit 121B measures
a communication rate of radio communication with the radio terminals 200.

[0080] In step S201, the reception unit 112 receives a measurement result
report from the radio terminals 200.

[0081] Firstly, the case in which the radio base station with the good
radio quality is the GSM base station 500 will be described. When the
radio base station with the good radio quality is the GSM base station
500 and the communication rate measured by the communication rate
measurement unit 121B is a low speed (step S203; YES), the handover
command generation unit 122 generates a handover command for a handover
toward the GSM base station 500 in step S206. The transmission unit 111
transmits the handover command generated by the handover command
generation unit 122 to the radio terminal 200.

[0082] Meanwhile, when the radio base station with the good radio quality
is the GSM base station 500 and the communication rate measured by the
communication rate measurement unit 121B is not the low speed (step S203;
NO), the handover command generation unit 122 omits the generation of the
handover command and the transmission unit 111 omits the transmission of
the handover command.

[0083] Secondly, the case in which the radio base station with the good
radio quality is the CDMA base station 400 will be described. When the
radio base station with the good radio quality is the CDMA base station
400 and the communication rate measured by the communication rate
measurement unit 121B is a medium speed (step S204; YES), the handover
command generation unit 122 generates a handover command for a handover
toward the CDMA base station 400 in step S206. The transmission unit 111
transmits the handover command generated by the handover command
generation unit 122 to the radio terminal 200.

[0084] Meanwhile, when the radio base station with the good radio quality
is the CDMA base station 400 and the communication rate measured by the
communication rate measurement unit 121B is not the medium speed (step
S204; NO), the handover command generation unit 122 omits the generation
of the handover command and the transmission unit 111 omits the
transmission of the handover command.

[0085] Thirdly, the case in which the radio base station with the good
radio quality is the LTE base station 300 will be described. When the
radio base station with the good radio quality is the LTE base station
300 and the communication rate measured by the communication rate
measurement unit 121B is a high speed (step S204; YES), the handover
command generation unit 122 generates a handover command for a handover
toward the LTE base station 300 in step S206. The transmission unit 111
transmits the handover command generated by the handover command
generation unit 122 to the radio terminal 200.

[0086] Meanwhile, when the radio base station with the good radio quality
is the LTE base station 300 and the communication rate measured by the
communication rate measurement unit 121B is not the high speed (step
S204; NO), the handover command generation unit 122 omits the generation
of the handover command and the transmission unit 111 omits the
transmission of the handover command.

(2.3) Operation and Effect

[0087] When the communication rate of the radio communication with the
radio terminals 200 is low, the LTE base station 100 allows the radio
terminal 200 to be handed over to the CDMA system or the GSM system. When
the communication rate of the radio communication with the radio
terminals 200 is high, the LTE base station 100 may allow the radio
terminal 200 to be handed over to the LTE system.

[0088] In this way, it is possible to reduce the load of the LTE base
station 100 when the radio terminal 200 (that is, the radio terminal 200
using a low speed communication service) performing low speed radio
communication is connected to the LTE base station 100. The load of the
LTE base station 100 is reduced, resulting in the improvement of service
quality when the radio terminal 200 (that is, the radio terminal 200
using a high-speed communication service) performing high speed radio
communication is connected to the LTE base station 100.

[0089] Consequently, according to the second embodiment, it is possible to
achieve the same effect as the first embodiment.

(3) Third Embodiment

[0090] In the third embodiment, the LTE base station 100 performs control
such that an offset value is added to radio quality measured in the radio
terminal 200 according to the classification of a communication service
being used by the radio terminal 200.

[0091] In the third embodiment, (3.1) Configuration of LTE Base Station,
(3.2) Operations of LTE Base Station and Radio Terminal, and (3.3)
Operation and Effect will be described, and the description overlapping
the first embodiment will be omitted.

(3.1) Configuration of LTE Base Station

[0092]FIG. 8 is a block diagram illustrating the configuration of the LTE
base station 100 according to the third embodiment.

[0093] As illustrated in FIG. 8, the third embodiment is different from
the first embodiment in that the LTE base station 100 further includes a
measurement control unit 123 and a handover determination unit 124.

[0094] The measurement control unit 123 controls the measurement of radio
quality in the radio terminal 200. In detail, the measurement control
unit 123 acquires an offset value associated with a radio communication
system from the storage unit 130 according to the classification of the
communication service being used by the radio terminal 200, which has
been detected by the service detection unit 121A. Furthermore, the
measurement control unit 123 controls the transmission unit 111 such that
the transmission unit 111 transmits measurement control information
(Measurement Configuration) including information indicating the acquired
offset value to the radio terminal 200.

[0095]FIG. 9 is a diagram illustrating a configuration example of the
table held by the storage unit 130.

[0096] As illustrated in FIG. 9, an offset value a related to radio
quality in the GSM system is associated with the voice communication
service. An offset value b related to radio quality in the CDMA system is
associated with the WEB browsing service using a protocol such as HTTP.
An offset value c related to radio quality in the LTE system is
associated with the file transfer service using a protocol such as FTP.
Furthermore, an offset value d related to radio quality in the LTE system
is associated with the case in which a plurality of services are
simultaneously used. In addition, the offset values a to d may be equal
to each other or may be different from each other.

[0097] The handover determination unit 124 determines whether to perform a
handover based on the measurement result report received in the reception
unit 112. For example, the handover determination unit 124 compares radio
quality between the radio terminal 200 and the LTE base station 100 with
radio quality between the radio terminal 200 and another radio base
station. When the radio quality between the radio terminal 200 and the
another radio base station is higher than the radio quality between the
radio terminal 200 and the LTE base station 100, the handover
determination unit 124 determines that the LTE base station 100 allows
the radio terminal 200 to be handed over to the another radio base
station. Here, an offset value supporting the classification of the
communication service is added to the radio quality included in the
measurement result report.

[0098] The handover command generation unit 122 checks the acceptance or
refusal of the radio terminal 200 with respect to another radio base
station selected as a handover destination by the handover determination
unit 124, and then generates a handover command for a handover toward the
another radio base station. The transmission unit 111 transmits the
generated handover command to the radio terminal 200.

(3.2) Operations of LTE Base Station and Radio Terminal

[0099]FIG. 10 is a sequence diagram illustrating an operation sequence of
the LTE base station 100 and the radio terminal 200 according to the
third embodiment. The operation sequence illustrated in FIG. 10 is
performed for each of the plurality of radio terminals 200 connected to
the LTE base station 100.

[0100] In step S301, the service detection unit 121A detects the
classification of a communication service being used by the radio
terminal 200 by using the method described in the first embodiment.

[0101] In step S302, the measurement control unit 123 acquires an offset
value from the storage unit 130 according to the classification of the
communication service being used by the radio terminal 200, which has
been detected by the service detection unit 121A. For example, when the
classification of the communication service being used by the radio
terminal 200 is the voice communication service, the measurement control
unit 123 acquires the offset value a related to the radio quality in the
GSM system. When the classification of the communication service being
used by the radio terminal 200 is the WEB browsing service, the
measurement control unit 123 acquires the offset value b related to the
radio quality in the CDMA system. When the classification of the
communication service being used by the radio terminal 200 is the file
transfer service, the measurement control unit 123 acquires the offset
value c related to the radio quality in the LTE system. When a plurality
of communication services are being used by the radio terminal 200, the
measurement control unit 123 acquires the offset value d related to the
radio quality in the LTE system. Then, the measurement control unit 123
generates the measurement control information (Measurement Configuration)
including information indicating the acquired offset values, and outputs
the measurement control information to the transmission unit 111.

[0102] In step S303, the transmission unit 111 transmits the measurement
control information (Measurement Configuration) to the radio terminal
200. The radio terminal 200 receives and stores the measurement control
information (Measurement Configuration).

[0103] In step S304, the radio terminal 200 measures radio quality in a
plurality of radio communication systems including the LTE system. For
example, the radio terminal 200 measures RSSIs or SINRs, which are
transmitted by the LTE base station 100, the LTE base station 300, the
CDMA base station 400, and the GSM base station 500, as radio quality.
Specifically, the RSSI is reference signal received power (RSRP) and the
SINR is reference signal reception quality (RSRQ).

[0104] In step S305, the radio terminal 200 adds an offset value, which is
included in the measurement control information (Measurement
Configuration), to the measured radio quality.

[0105] In step S306, the radio terminal 200 transmits a measurement result
report (Measurement Report), which includes information indicating the
radio quality in the plurality of radio communication systems, to the LTE
base station 100. Here, the radio quality has an offset value added
thereto. The LTE base station 100 receives the measurement result report
(Measurement Report).

[0106] In step S307, the LTE base station 100 compares radio quality
between the radio terminal 200 and the LTE base station 100 with radio
quality between the radio terminal 200 and another radio base station.
When the radio quality between the radio terminal 200 and the another
radio base station is higher than the radio quality between the radio
terminal 200 and the LTE base station 100, the handover determination
unit 124 determines that the LTE base station 100 allows the radio
terminal 200 to be handed over to the another radio base station.

[0107] The LTE base station 100 checks the acceptance or refusal of the
radio terminal 200 with respect to the another radio base station
selected as a handover destination by the handover determination unit 124
(steps S308 and S309), and then generates a handover command (Handover
Command) for a handover toward the another radio base station in step
S310. The transmission unit 111 transmits the generated handover command
(Handover Command) to the radio terminal 200.

(3.3) Operation and Effect

[0108] The LTE base station 100 controls the measurement in the radio
terminal 200, thereby allowing the radio terminal 200 to be
preferentially handed over to the CDMA system or the GSM system when the
communication rate required in the classification of the communication
service being used by the radio terminal 200 is low. The LTE base station
100 controls the measurement in the radio terminal 200, thereby allowing
the radio terminal 200 to be preferentially handed over to the LTE system
when the communication rate required in the classification of the
communication service being used by the radio terminal 200 is high.

[0109] In this way, it is possible to reduce the load of the LTE base
station 100 when the radio terminal 200 having used a low speed
communication service is connected to the LTE base station 100. The load
of the LTE base station 100 is reduced, resulting in the improvement of
service quality when the radio terminal 200 having used a high-speed
communication service is connected to the LTE base station 100.

[0110] As described above, it is possible to allow a radio resource of the
LTE system to be preferentially used by the radio terminal 200 having
used the high-speed communication service, so that the radio resource of
the LTE system can be effectively utilized. Moreover, handover from the
LTE system to an existing radio communication system (the CDMA system or
the GSM system) is actively performed, so that load distribution can be
more flexibly achieved.

(4) Other Embodiments

[0111] While the present invention has been described by way of the
foregoing embodiments and examples, as described above, it should not be
understood that those descriptions and drawings constituting a part of
the present disclosure limit the present invention. Further, various
substitutions, examples or operational techniques shall be apparent to a
person skilled in the art based on this disclosure.

(4.1) First Modification

[0112] In the above-mentioned first embodiment and the second embodiment,
the voice communication service, the WEB browsing service, and the file
transfer service have been described as the classification of a
communication service. However, the present invention is not limited to
these communication services.

[0113] For example, a communication service requiring a high communication
rate includes a moving image streaming service, a bi-directional game
application and the like, in addition to the file transfer service. A
communication service requiring a low communication rate includes a mail
service and the like, in addition to the voice communication service.

(4.2) Second Modification

[0114] In the above-mentioned first embodiment and the second embodiment,
the GSM system and the CDMA system have been described as a handover
destination from the LTE system. However, the present invention is not
limited to the GSM system and the CDMA system. That is, the handover
destination may also include other radio communication systems (for
example, WiMAX (a registered trademark), iBurst (a registered trademark)
and the like).

[0115] Moreover, a radio base station transmitting the handover command is
the LTE base station 100. However, the present invention is not limited
to the LTE base station 100 as long as it is a radio base station of a
radio communication system employing handover led by a base station.

(4.3) Third Modification

[0116] In the above-mentioned first embodiment and the second embodiment,
RRM set forth by the LTE specifications has not been specifically
mentioned. However, it may be possible to use the handover control method
according to the first embodiment and the second embodiment, together
with the RRM set forth by the LTE specifications. The RRM includes RBC
(Radio Bearer Control), RAC (Radio Admission Control), CMC (Connection
Mobility Control), DRA (Dynamic Resource Allocation), ICIC (Inter-cell
Interference Coordination), Load Balance (LB), Inter-RAT Radio Resource
Management, Subscriber Profile ID for RAT/Frequency Priority, and the
like.

(4.4) Fourth Modification

[0117] In the above-mentioned first embodiment, the LTE base station 100
decides a radio base station (a handover destination) according to the
classification of the communication service being used by the radio
terminal 200 and the radio quality. In the above-mentioned second
embodiment, the LTE base station 100 decides a radio base station (a
handover destination) according to the communication rate of radio
communication with the radio terminal 200 and the radio quality. However,
when the LTE base station 100 has already comprehended a peripheral radio
base station, it may be possible to decide a radio base station (a
handover destination) without considering radio quality.

(4.5) Fifth Modification

[0118] In the above-mentioned third embodiment, the case in which the
offset value is added according to the classification of the
communication service is described. However, such a method may be applied
to the second embodiment and the offset value may be added according to
the communication rate of the radio communication.

(4.6) Sixth Modification

[0119] In the above-mentioned third embodiment, the LTE base station 100
asks another radio base station about the acceptance or refusal of the
radio terminal 200 before transmitting the handover command to the radio
terminal 200. However, such a method may also be applied to the first
embodiment and the second embodiment.

[0120] Thus, it must be understood that the present invention includes
various embodiments that are not described herein. Therefore, the present
invention is limited only by the specific features of the invention in
the scope of the claims reasonably evident from the disclosure above.

[0121] The entire contents of Japanese Patent Application No. 2009-174697
(filed on Jul. 27, 2009) are incorporated in the present specification by
reference.

INDUSTRIAL APPLICABILITY

[0122] As described above, in accordance with the radio base station and
the handover control method according to the present invention, when a
plurality of radio communication systems exist, since it is possible to
effectively utilize a radio communication system capable of performing
high-speed communication, it is available for radio communication such as
mobile communication.